The present invention relates to a rotational control apparatus and more particularly to an eddy current fan drive system operable to drive a cooling fan with multiple speed control.
In a typical eddy current drive, a first magnetic field producing drive component, such as in the form of permanent magnets, is mounted for rotation with either the input or output of a rotational control apparatus, such as a clutch, for magnetically coupling with a second coupling drive component mounted for rotation with either the input or output. The second coupling drive component was typically in the form of a first ring formed of electrically conductive material, such as copper, sandwiching a second ring of magnetic flux conductive material, such as steel, against a mounting body portion formed of heat conductive material, such as aluminum, so that the body portion acts as a heat sink. Screws extend through the first and second rings and are threaded into the body portion.
These coupling drive components suffered from various problems and deficiencies. The mechanical attachment of the steel and copper rings to the body portion increases the number of components and assembly required and is subject to loosening and/or separating and thereby detrimentally affecting reliability. Also, the first and second drive components are bearing mounted and fixed close to a fan. The first and second drive components are axially arranged face to face with a fixed gap; there is no relative axial movement between the drive components. Such an arrangement provides two speed fan drives. Current two speed cooling fan customers require more drive torque to rotate larger fans and to rotate fans at higher speeds. Current eddy current drive systems used to operate cooling fans do not have the torque capacity to handle these applications.
Fan drives exist that can handle the torque required by larger fan applications, however, these fan drives do not include a desired second intermediate speed. One fan clutch that has a large torque capacity is disclosed in U.S. Pat. No. 6,092,638, entitled “Splineless Rotational Control Apparatus” and assigned to Horton, Inc. (the assignee of the present application).
The fan clutch includes an actuation system to apply an axial load on a coupling element. The actuation system is generally constituted of either one of the rotatable components or a separate part, to create some relative axial movement between the coupling elements and fan mounted friction disc (FMFD) (one of the rotatable components) to drivingly interconnect the two rotatable components. Typically, the coupling element is an annular piston which is acted upon by at least one spring to bias the piston toward an activated or a deactivated position and move the FMFD.
In a typical axially arranged eddy current fan drive there is a fixed gap because the two assemblies that create the eddy current coupling are bearing mounted and fixed to the journal shaft. The eddy current drive may be adapted to a large torque capacity fan clutch to allow relative axial movement between the two assemblies. In the axially actuated fan clutch the magnet poles of the eddy current assembly lie along a line parallel to the central axis and located at a fixed radial distance from the axis. The eddy current assembly includes a non-magnetic material with an axially adjacent magnetic material which is axially spaced from the magnets by an air gap. The air gap between the two assemblies is not fixed because the FMFD moves axially and the two eddy current coupling assemblies move axially relative to one another. Thus, an eddy current coupling is needed that maintains a constant gap while allowing axial relative movement between the two assemblies.
The axially actuated fan clutch is used to selectively control the transmission of rotational forces between first and second relatively rotatable members, and in particular to a frictional clutching device for driving a fan. The axially actuated fan clutch can handle the drive torque necessary to rotate larger fans and to rotate fans at higher speeds. However, the axially actuated fan clutch typically are one speed and drive the fan between an on and off position.
Eddy current drives typically produce torque producing eddy currents and non-torque producing eddy currents. When a non-torque producing eddy current is created (an eddy current which does not transmit torque) it is a loss, an inefficiency, and a source of increased heat within the drive. Eddy current drives utilizing a solid-iron back iron as the magnetic material produce the non-torque producing eddy currents. The elimination of non-torque producing eddy currents increases the inefficiency of the eddy current drive.
Thus, an eddy current drive is needed that eliminates the non-torque producing eddy currents, enhances the production of torque producing eddy currents (torque producing eddy currents which are desired to improve the torque capacity of the fan drive and operate the fan at higher speeds), and can handle the torque required to rotate larger fans, to rotate fans at higher speeds and to permit multiple, but fixed, speed operation.